1. Technical Field of Invention
The present invention relates to a processing apparatus for fabricating LSI in which a predetermined gas is supplied into a chamber to conduct an intended processing on a substrate. More particularly, the present invention relates to a processing apparatus in which laser light is guided into a chamber from the outside of the chamber through a window of the chamber in order to observe an occurrence state of a fine particle which adversely affects the intended processing and measure scattered light from the fine particle floating in the chamber.
2. Related Art
When laser light is guided in a chamber through a window of the chamber and scattered light from a fine particle (for example, dust or dirt) in the chamber is measured by a variety of causes stray light occurs other than the scattered light from the fine particle. The stray light comprises: reflecting light on front and rear surfaces of the window from the laser light, irregularly reflecting light on the inside wall of the apparatus, irregularly reflecting light on a stopper of the laser light, and the like. The other reflected lights are usually stronger than a weak fine particle scattered light. Therefore, in order to measure such weak scattered light from a fine particle, it is desirable to suppress the reflecting light and irregularly reflecting light. As a method to realize this, prior art has generally adopted, an antireflection coat applied on a window (hereinafter referred to as incident side window) through which laser light is guided in order to reduce a reflectance or a beam stopper made black in order to suppress irregularly reflecting light.
Alternatively for a method in which multiple reflection light of laser light by the incident side window or irregularly reflecting light from a light stopper for laser light is reduced includes a beam damper called a baffle, a Rayleigh's horn or the like. In this regard, a summary is described in Tadao Tsuruta "light pencil" New Technology Communication Co. 1984; pp 296 to 306.
While the above mentioned methods are effective, two problems arise when for example, a chemically active gas (hereinafter referred as active gas) is supplied into a chamber and a film formation process is conducted on a semiconductor substrate by chemical reaction.
One is a problem that occurs on an incident side window of laser light. That is, since the chamber inside surface of the incident side window is exposed to an active gas, a reaction product deposits on the window due to a chemical reaction of the active gas with the window material. As a result, scattered light is produced by the deposited product, which scattered light hinders measurement on that essentially desired to be measured weak scattered light caused by a fine particle. Besides, an intensity of laser light to be guided into the chamber is reduced due to scattering by the deposited product and thereby an intensity of the weak scattered light from the fine particle is weaker, which makes measurement harder. When an intensity of laser light is strong, the deposited product is sometimes scattered to contaminate the inside of 10 the chamber.
The other problem occurs on a stopper for laser light. When laser light is guided into the chamber, a beam damper is mounted on the inside wall of the chamber opposed to the incident side window as a stopper for the laser light. The damper is ideally fabricated with a 100% material absorbing light. Since a material has necessarily a reflectance which is not zero, however, a Rayleigh's horn or a bundle of razor's blades, as described in the above mentioned reference, is used to achieve a reflectance of close to zero by use of multiple reflections. In this case, since surface area is increased, there arises the same problem as on the chamber inside surface of the incident side window as has been mentioned. That is, light scattering on the damper surface is increased due to deposition of a reaction product or a deposited product on the damper surface is scattered around by strong laser light, which contaminates the inside of the chamber.
Thus, when weak scattered light from a fine particle in the chamber is measured in a processing apparatus in which a chemically active gas is used, it is insufficient only to use a baffle, a Rayleigh's horn or a bundle of razor's blades, which have conventionally been used. This is a problem as detecting fine particles in a processing apparatus chamber is an important means to stabilize a processing condition of a substrate.